ANODIC OXIDATION OF FORMIC ACID

 

Gojo M. Karšulin M.

 

In this paper an anodic oxidation of for-mic acid on rhodinized platinum elec-trodes, with and without pyridine addi-tion is investigated. During anodic oxi-dation of HCOOH, in the potential ran-ge from 600 to 1000 mv(SHE), periodic oscillations occurred (Fig. 1a).

As the speed of polarization is an essential for the oscillations occurrence; the speed at the rate of 0,3 mVs l was used. The oscillations disappear at higher speeds of polarization.

We can therefore propose the following formulation for the anodic behaviour of formic acid on Pt (Rh) anodes:

 

Rh° → Rh²⁺ + 2e^-                                           (1)

 

(HCOOH)₂ → 2 HCOOH                    (2)

 

Formic acid exists as dimer, and during the contact with Rh, a monomerization occurs.

 

2 HCOOH → 2 HCOO^- + 2H⁺ ( 3 )

 

2 H⁺ goes to cathode and gives H₂.

 

where Rh(OOCH)₂ represents anodic layer: Further:

 

Rh(OOCH)₂ → Rh° + H₂ + 2 CO₂       (5)

 

 

 

 

 

 

According to literature data it is po-ssible to synthesise a stable complex of Rh(OOCH)₂L₂, where as a ligand is either pyridine or NH₃. By pyridine addition into formic acid a decom-position of Rh(OOCH)₂ into CO and H₂, is prevented, although it caused a periodic oscillations (Fig. 1b).

Our study gives an explanation for the mechanism of the HCOOH anodic be-haviour that is possible without the assumption of problematic radicals use, i.e. without very unstable Rh⁺. The interpretation of the mechanism of ano-dic process through Rh oxidation is also unlikely to occur, as the oxide formation occurs only at the very positive potential values.